improve margin support

[iaacornus]

currently, the margin support is only for all side (that is all sides will be equal). In the argument from cli, it should be able to array an array of values that will be used for margin:

[top, bottom, left, right]

then, the argument can be: -m=[1.5, 1, 1.5, 1], or it can be specified in the parameter in simtex.json

"MARGIN": [1.5, 1, 1.5, 1]

which would give a margin of top=1.5, bottom=1, left=1.5 and right=1 using geometry package.